Hello Lothars Sohn,
1.) “do you know Calvinists who don’t believe in Biblical or are rather liberal while still holding fast on TULIP?”
I don’t know of any, I think if there’s a Calvinist who doesn’t believe their Bible or are rather Liberal, their Liberalness would eventually undermine and erode their Calvinism/TULIP.

2.) “What are the theological positions of these writers of Joel’s commentary?”

The commentaries I have read on Joel (though I didn’t necessarily finish them with this one as an exception) span the theological range (Liberal-Conservative). As a preacher I have something to appreciate from them all, though with discernment. Most commentators on Joel seems to be Evangelical Protestants (or maybe that’s what I stumble upon most). Irv Busenitz, the author of this volume I review, would be a Conservative Evangelical.